Process for producing shaped thin articles from metal powder



M. D. AYERS June 27, 1967 PROCESS FOR PRODUCING SHAPED THIN ARTICLES FROM METAL POWDER Filed Oct, 27, 1964 xxx INVENTOR Maurice D. Ayers BY W, Wad Wm M 1' (1M ATTORNEYS United States Patent C) l 3,328,166 PROCES FUR PRODUCING SHAPED THIN ARTKCLES FROM METAL POWDER Maurice D. Ayers, Metals Innovations Inc., 121 Woodside Drive, Greenwich, Conn. 06830 Filed Oct. 27, 1964, Ser. No. 406,805 8 Claims. (Cl. 75-214) This invention relates to the production of relatively thin strip and sheets of metal from metal powder, and has for its object the provision of an improved process for producing shaped articles of metal sheets which achieves a material saving in the cost of the articles in comparison with similar articles made by present practices. In rolling operations, continuous sheets are usually called strips, but herein, for convenience, the word sheet will define rolled products of any length, whether continuous or in various lengths or shapes.

The process is applicable to the production of the sheets from such metal powders as iron, copper, nickel, aluminum, cobalt, etc. or alloys thereof which may be in any desired degree of fineness and comprises several processing steps including the important step of initially compacting the powder into so-called green sheets which can be handled and cut trimmed or stamped when necessary to form articles which are then passed through another rolling operation for further compacting as desired to form a strong and serviceable article. The initial compacting gives a sheet which is sufliciently coherent that it may be cut to form sheet parts for further compacting leaving other parts or excess material for recovery. The invention is particularly advantageous in the production of articles formed of iron sheets for use in the manufacture of small motors, transformers, solenoids, and the like, or rings for automotive transmissions.

For example, in stamping laminations for motors or rings for automatic transmissions much metal of the sheet is scrap which results in a reduced yield of prime material from starting material. My invention utilizes this excess material after initial compacting as it can easily be reduced to powder and sent back to the operation for further compacting to form more green sheets. The selected portions of sheet are subjected to further processing to produce the final article.

The invention in its more complete aspects comprises a series of processing steps including the compacting of powdered metal to form a coherent sheet which can be handled but which can easily be reduced to powder, further compacting of the sheet in one or more stages of rolling to form a sheet to any desired density up to and including 100%. The initially compacted sheet is preferably given any desired heat treatment in any desired atmosphere and hot rolled to final density.

The powder may be in any desired state of subdivision, for example, a product that will all pass through a 40 mesh screen. It is preferred to use particles of irregular shape which form a stronger green sheet and a more easily produced final product. The powder may be produced by various means as by precipitation, grinding or atomizing. A particularly effective type of powder is produced by the process of my patent application Ser. No. 321,246, filed Nov. 4, 1963, now US. Patent No. 3,281,- 893. It is advantageous to use powders consisting of various sizes since the finer particles pack in between the larger particles and facilitate a compacting of the particles into a coherent dense sheet. The powder may be blended with other alloying components and may be oxidized, reduced or annealed as desired.

In carrying out a typical operation of the invention, using, for example, atomized iron or steel powder low in carbon (below 0.25), the powder or blended powder may 3,328,166 Patented June 27, 1967 be stored in bins from which it is fed continuously at a; controlled rate and distribution between a first stage of compacting rollers which compresses the powder into a so-called green strip which is self-supporting, although weak, and has a preferred density of about 70 to percent.

This sheet may vary in thickness say from 0.010 inch to 0.250 inch, and in width from 1 inch to 75 inches. The resulting green sheet is passed continuously into the next operation which consists in cutting, trimming or punching it into sheet articles desired for further processing, and the excess material. This material is separated from the remainder of the sheet and is easily shattered into powder. Preferably it is sent to a ball mill wherein it is powdered and returned to the original supply.

The selected portion of the compacted green sheet (continuous or severed sections) is directed into a special heating chamber, in which the green strip is brought up to a higher temperature sufficient to enable a second stage of compacting to be carried out, to reduce the sheet to substantially percent density. In addition, the heating chamber is provided with means for subjecting the metal to various reactive treatments, since the metal is still in a highly porous form. Such reactions as carburizing, decar burizing, deoxidation, nitriding, chromatizing, nickelizing, etc., may be carried out wit-h high efliciency, because of the porous nature and high area exposure of the partially compacted metal. In addition, and or particular practical significance, it is possible at this stage to infiltrate the porous metal with dissimilar metals of lower melting point, such as the infiltration of porous iron with copper, for example.

When compacting to substantially 100 percent density, the sheet has substantially conventional characteristics. Because it is at an elevated temperature at this point, and it is still subject to the protection of the controlled atmosphere, the sheet is additionally hot-rolled in one or more stages to a desired gauge, which typically could be well below the conventional hot-rolled gauges, because the starting strip thickness is considerably less than in the case of slabs used for conventional strip-rolling procedures.

After cooling to reduce the likelihood of oxidation, the strip is brought out into the open atmosphere, subjected to such optional treatments as may be appropriate, such as cold-rolling to impart desired surface characteristics or temper, and then separated, as desired.

The single figure of the accompanying drawing is a simplified, schematic arrangement of apparatus for carry ing out a process of the invention.

In the continuous procedure of the invention, the dried metal powder which has been suitably processed and screened is stored in the holding bin 1, the latter being supplied with controlled atmosphere, such as air with a dew point of 40 F. or less, as through an inlet conduit 2. The holding bin provides for fluctuations in the rate of powder making and the rate of subsequent processing into sheets, as will be understood.

Associated with the outlet 3 of a holding bin is a blending chamber 4, in which the metal powder particles may be mixed and blended with desired additives, such as detergents, activators, lubricants, binders, or, in appropriate cases, other metal powders or alloying agents. The additives typically may be introduced through an inlet facility 5.

The blending operation may be of particular significance because of the advantageous controls provided over the final metal composition, as well as the ability to promote or facilitate certain of the subsequent operations. For example, the addition of appropriate detergents and activators can significantly reduce the times and temu peratures required for subsequent heating and/ or sintering operations. Further, various desirable lubricants and binders, can greatly facilitate the operation of compacting the powder to form a green sheet. Of perhaps even greater importance, however, the blending stage permits alloying powders to be mixed with the otherwise high purity metals to achieve a variety of advantageous effects, including the formation of alloys otherwise impossible or impractical to produce.

By way of example, in the production of controlled analysis steel sheets in accordance with the invention, the iron or steel powder should be as soft as practicable. This is obtained by reducing the carbon content to about 0.10 percent or less, and/or annealing the powder so that the basic powder would be as soft as possible for proper subsequent compacting. At the blending stage, desirable percentages of graphite or high carbon steel powder may be blended with (the softer) iron powder, so that the desired average amount of carbon is present in the final steel material. In this respect, while the formation of a metal sheet directly from a higher carbon content steel powder would present substantial difiiculties, because of hardness of the pow-der, such ditficulties are effectively avoided by blending the softer iron and high carbon steel powders to achieve a desired average carbon content in the final product.

Another particularly advantageous blending procedure which may be followed in the process of the invention is the alloying with iron or steel of relatively high percentages of copper, which can result in significantly increased tensile strength and fatigue endurance of the final product, as well as substantial improvements in its corrosion resistance. By conventional steel making practices, it has not been practicable to utilize copper as an alloying constituent, because of a tendency of the copper to separate out upon heating of the steel. By contrast, in accordance with the process of the invention, virtually any percentage of copper may be added to the metal in the blending step (or in a subsequent infiltration procedure to be described), and true alloy characteristics are realized in the final material.

As may be appreciated from the foregoing, the blending stage offers an opportunity for the convenient preparation of an extremely wide variety of alloy combinations, enabling an extraordinarily wide range of end products to be produced. Further the blending of various compositions may be carried out efi'iciently on a small quantity basis, so that the metal products may economically be prepared especially for particular end uses. The metal powder is controllably and continuously fed to the compacting rollers 7. Precise fee-d control is important in order to achieve a uniform rate of feed toward the compacting rollers 7 and to assure that the rate is uniform across the entire width of the compacting rollers. Where iron or steel making powder blends are employed in the procedure, the feed control facility (not specifically illustrated) may include appropriate magnetic pump or roller means, for example.

In the process of the invention, the powder particles are compacted by the rollers 7 to a density in the range of 70 to 90 percent that of solid metal and advantageously this is accomplished using compacting rollers having a diameter greatly in excess of the compacted sheet thickness (for example in the order of 100 to 300 times the thickness of the initially compacted sheet). The product emerging from the first stage of the compacting rollers 7 is referred to as a green strip 8. It is reasonably integrated or coherent and is self-supporting but is still quite weak relative to finished metal strip. It can be shattered and again reduced to the original powder state.

Following initial compaction, the green strip is directed into a fabricating unit 10 which may include shears, punches and dies or rollers with cutting profiles and the like, to cut, stamp or otherwise form articles of the desired shape for further processing. This severance of the desired articles and their separation from the remaining sheet leaves varying amounts of the green sheet as excess material. This material is removed and sent to the ball mill 11 wherein it is reduced to powder and returned to the storage bin 1. This results in an efficient utilization of metal powder in a very economical manner. The selected formed articles or trimmed sheet is passed into the elongated heating chamber 12, in which the green sheet articles as individual parts or as parts still in the continuous sheet is heated to a higher temperature in the range of 1600 F. to 2200F. The green sheet or sheet of articles which may be partially sintered in the heating chamber 12, is in any event in a desirably heated condition for further compacting, such as by rolling or pressing. A combination of pressing and rolling or either alone may be used to increase the density of the sheet after it leaves chamber 12. Suitable stamping equip ment may also be used. Rolling can be by compacting sets of rollers 13 and 15 which serve to compact the heated strip to substantially percent density.

The strip passing through the heating chamber 12, being in a porous condition and at high temperature is ideally receptive to a variety of gas reaction treatments, such as carburizing, decarburizing, deoxidation, nitriding, etc. These reaction treatments may be advantageously carried out by introducing appropriate gases into the heating chamber or into selected, divided regions of the heating chamber. In this connection, the chamber may be made as long as is necessary and desirable to effect the necessary heating of the strip and its exposure to the reacting medium. Further, advantage may be taken of the heated, porous condition of the green strip within the heating chamber to cause the strip to be infiltrated with a lower melting point metal. Iron or steel strip, for example, may be readily infiltrated with molten copper, such that the product emerging from the heating chamber is a substantially solid material of unique properties. Various additives from the blending stage also bring about advantageous effects. Detergents and activators promote sintering or hot compacting, and compounds such as dissociable hydrides release protective or treating gases in the immediate vicinity of the particles.

In the production of thin articles such as iron sheet articles in accordance with the invention, it is contemplated that the densified sheet 14 will be reduced to a substantially finished size or thickness, and one or more hot roll reduction stages 16 advantageously are provided for this purpose, located immediately following the compacting rollers 13 and 15, to receive the densitied metal while it still retains the heat from the chamber 12. After the sheet passes beyond rollers 16 it may be passed through cooling sprays 17 to cool it to a temperature below 400 F.

In accordance with one aspect of the invention, the iron or other powder may be maintained under a controlled atmosphere from the time of its delivery as green strip sheets to the heating chamber 12 to the time of its emergence beyond sprays 17 as a substantially finished product at a temperature below that at which oxidation will readily occur. To this end, it is appropriate to maintain the sheet wholly enclosed in a suitable chamber 18 (or series of chambers) which, in effect at least, embraces the sheet from the point of its initial heating to the point of its emergence at a relatively low temperature. The: chamber 18 is supplied, as through 19, with a suitable atmosphere, such as hydrogen, nitrogen, carbon monoxide, etc. In this connection, it may be desirable to embrace the sheet with a series of individual chambers, rather than a single large chamber, to achieve various practical conveniences.

The sheet 14 may be protected from oxidation as it travels from the furnace 12 to the cooling sprays 17 by flame curtains, which are reducing. However, it may be desirable in some cases to impart a controlled oxide coating on the sheet surface.

The cool, substantially finished articles may be given any desired further treatment such as temper rolling, further dimensioning, shaping or polishing at 20.

I claim:

1. The continuous process for producing sheet metal articles from metal powder which comprises feeding to compacting rollers a powdered metal and compacting the powder to form a green sheet which is coherent but which can be repowdered, cutting from the green sheet articles of a desired shape for further processing leaving part of the sheet as excess material, separating this material and mechanically reducing it to powder, returning the reduced powder to the process for forming more green sheet, and passing the articles to a heating operation.

2. In the process of claim 1, compressing the sheet articles to higher density as in rolling operation.

3. In the process of claim 1 forming the sheet of a metal powder of the group consisting of iron, copper, aluminum, nickel, titanium, molybdenum, cobalt, and alloys thereof.

4. In the process of claim 1 forming the green sheet into articles of approximate final shape, and passing them in a continuous uninterrupted movement to a hot rolling operation.

5. In the process of claim 4 forming the articles by one of the operations consisting of cutting, stamping and pressmg.

6. In the process of claim 1 cutting from the sheet excess material leaving a cut sheet for further processing and mechanically reducing the excess material to powder in a ball mill.

7. The continuous process for producing dense sheet metal from metal powder which comprises feeding metal powder to a compacting operation and compacting the powder to form a green sheet which is coherent but which can be repowdered, severing from the green sheet excess material leaving the remaining green sheet for further processing, reducing the excess material to powder, returning the powder to the process for forming more green sheet, heating the green sheet from which excess material was severed and compressing it to higher density which is substantially the density of the unpowdered metal.

8. In the process of claim 7 forming the sheets of steel powder and hot rolling the steel to approximately density.

References Cited UNITED STATES PATENTS CARL D. QUARFORTH, Primary Examiner.

R. L. GRUDZIECKI, Assistant Examiner. 

1. THE CONTINUOUS PROCESS FOR PRODUCING SHEET METAL ARTICLES FROM METAL POWDER WHICH COMPRISES FEEDING TO COMPACTING ROLLERS A POWDERED METAL AND COMPACTING THE POWDER TO FORM A GREEN SHEET WHICH IS COHERENT BUT WHICH CAN BE REPOWDERED, CUTTING FROM THE GREEN SHEET ARTICLES OF A DESIRED SHAPE FOR FURTHER PROCESSING LEAVING PART OF THE SHEET AS EXCESS MATERIAL, SEPARATING THIS MATERIAL AND MECHANICALLY REDUCING IT TO POWDER, RETURNING THE REDUCED POWDER TO THE PROCESS FOR FORMING MORE GREEN SHEET, AND PASSING THE ARTICLES TO A HEATING OPERATION. 